Cyan, magenta, and yellow dyes that create photographic images fade with time, especially when exposed to various ambient lighting conditions such as sunlight, incandescent light, or fluorescent light. Most damage is usually done by UV-radiation that may be present in any lighting source. It is, therefore, desirable to make photographic products, especially photographic paper that is used to display images of both personal and commercial scenes, as stable as possible to fade. There are various means of achieving improved dye stability. Since products such as color paper are high volume products that are highly price sensitive, it is not always commercially feasible to replace an existing coupler with low cost with a new more stable and expensive coupler. Photographic paper structure, as shown in Table I, contains UV-absorbing compound dispersed in protective layers to absorb the damaging UV-radiation and prevent it from reaching the image dyes. Usually such UV-absorbing compounds have slight yellow coloration which, when applied in large enough quantities, cause the paper white areas to appear yellow, which is highly undesirable. Therefore, there is a limit to the extent such UV-absorbing materials could be applied in a photographic product such as paper.
TABLE I ______________________________________ Layer Structure of a Model Multilayer Color Paper System (Numbers indicate coverage in mg per square ft.) (Numbers within " " indicate same in mg per square meter) ______________________________________ LAYER-7 Overcoat: 125.0 Gelatin; "1336" 2.0 (SC-1) (Conventional Scavenger Dispersed in Solvent); "21" LAYER-6 UV Protection Layer: 61.0 Gelatin; "653" 34.3 Tinuvin 328 (Co-dispersed) Ultraviolet light absorber; "364" 5.7 Tinuvin 326 (Co-dispersed) Ultraviolet light absorber; "60" 4.0 (SC-1) (Co-dispersed in Solvent); "43" LAYER-5 Red Layer: 115.0 Gelatin; "1230" 39.3 (C-3) (Cyan Cplr. Co-dispersed in Solv.); "420" 0.5 (SC-1) (Scavenger Co-dispersed in Solvent); "5" 16.7 AgCl (In Red Sensitized AgCl Emulsion); "179" LAYER-4 UV Protection Layer: 61.0 Gelatin; "653" 34.3 Tinuvin 328 (Co-dispersed); "364" 5.7 Tinuvin 326 (Co-dispersed); "60" 4.0 (SC-1) (Co-dispersed in Solvent); "43" LAYER-3 Green Layer: 115.0 Gelatin; "1230" 41.5 (C-2) (Magenta Coupler Co-dispersed in Solvent); "444" 18.2 (ST-1) (Stabilizer Co-dispersed in Solvent.: "195" 3.4 (SC-1) (Scavenger Co-dispersed in Solvent); "37" 24.5 AgCl (In Green Sensitized AgCl Emulsion); "262" LAYER-2 Inter Layer: 70.0 Gelatin; "749" 9.0 (SC-1) (Scavenger Dispersed in Solvent); "96" LAYER-1 Blue Layer: 140.0 Gelatin; "1498" 100.0 (C-1) (Yellow Coupler Dispersed in Solv.); "1070" 30.0 AgCl (In Blue Sensitized AgCl Emulsion); "321" Resin Coat: Titanox Dispersed in Polyethylene (titanium dioxide) Support: Paper Resin Coat: Polyethylene ______________________________________ (Structures of compounds indicated in the text later)
Publications such as U.S. Pat. No. 4,283,486--Ano et al describe oxygen barrier layer comprising polyvinyl alcohol (PVA) that is a very low oxygen permeability, coated on photographic supports to prevent oxidative fade of photographic dyes. PVA has also been used in the photographic and as sizing material for photographic paper, U.S. Pat. No. 4,399,245--Kleber et al; also as subbing of photographic supports, U.S. Pat. No. 4,542,093--Suzuki et al; and in antistatic coatings, U.S. Pat. No. 4,770,487--Takahashi.
Oxygen barrier technology using coated PVA layer is considered to work well in multilayer photographic systems where the dyes of all the dye-forming couplers, UV absorbing materials and oxidized developer scavengers in all the layers fade by an ambient oxygen-oxidative mechanism. The dyes of some couplers undergo fade by a reductive mechanism. Therefore, unselective exclusion of oxygen by a universal oxygen barrier will tend to increase the fade of such dyes, of different color if present in the same photographic multilayer packet. Consequently, a selective mode of oxygen exclusion of the individual dyes in the individual layers is both preferred and necessary.
Conventional dispersions of coupler or other photographic addenda are usually prepared by dissolving the compound in a high boiling solvent and then dispersing it in water using a surfactant to stabilize the interface in the presence of the film forming well-known photographic steric stabilizer gelatin, which adsorbs on the surface of the coupler particles and prevents them from coalescence, as described in T. H. James in "The Theory of the Photographic Processes", 4th Edition, MacMillan, N.Y. (1977). Sometimes in such preparation of conventional dispersions, a low boiling water soluble auxiliary solvent is also used, which is washed out of the chilled dispersion or evaporated off after preparation of the dispersion. Various low and high boiling solvents useful in the preparation of photographic dispersions are given in U.S. Pat. No. 4,970,139 and U.S. Pat. No. 5,089,380 of Bagchi and coappended herewith.
There are many methods known in the art where microprecipitated dispersions can be prepared without gelatin present. It has been known in the photographic arts to precipitate photographic materials, such as couplers, from solvent solution. The precipitation of such materials can generally be accomplished by a shift in the content of a water miscible solvent (U.S. Pat. No. 4,933,270--Bagchi) and/or a shift in pH. The precipitation by a shift in the content of water miscible solvent is normally accomplished by the addition of an excess of water to a solvent solution. The excess of water, in which the photographic component is insoluble, will cause precipitation of the photographic component as small particles. The solvent shift method (U.S. Pat. No. 4,933,270--Bagchi) is particularly useful for couplers that are base degradable. In precipitation by pH shift, a photographic component is dissolved in a solvent that is either acidic or basic. The pH is then shifted such that acidic solutions are made basic or basic solutions are made acidic in order to precipitate particles of the photographic component which is insoluble at that pH. United Kingdom Patent 1,193,349--Townsley et al discloses a process wherein an organic solvent, aqueous alkali solution of a color coupler is mixed with an aqueous acid medium to precipitate the color coupler. In an article in Research Disclosure, December, 1977, entitled "Process for Preparing Stable Aqueous Dispersions of Certain Hydrophobic Materials", pages 75-80, by William J. Priest, it is disclosed that color couplers can be formed by precipitation of small particles from solutions of the couplers in organic auxiliary solvents. U.S. Pat. No. 4,990,431--Bagchi et al describes a three stream pH shift method for the manufacturing of microprecipitated dispersions in the absence of gelatin. For couplers that need permanent solvent for activity, a similar three stream pH shift method has also been described by Bagchi in U.S. Pat. No. 4,970,139 to obtain a gelatin-free coupler solvent containing microprecipitated coupler dispersions.
It has been shown that when coupler molecules are imbibed into latex particles by dissolving the coupler in a water-miscible solvent, adding this to the latex and removing the solvent, the resultant dispersion produces adequate photographic activity (Chen et al U.S. Pat. Nos. 4,199,363; 4,214,097; 4,133,687 and Tong U.S. Pat. Nos. 2,852,386; 2,772,163) for photographic utility. It seems that the polymer latex acts as a coupler solvent; however, such loading procedure requires very large quantities of solvent, which makes this procedure very expensive and somewhat hazardous for industrial production. In general, such procedure is limited to a load of 3 part coupler and 1 part latex polymer. Prior art (Takaharti European Application 0,256,531) indicates that polymerization or incorporation of a polymer into mechanically ground dispersions with no permanent solvent produces coupler dispersions that give very stable dye images. Also, incorporation of polymer into the photographic layer produces images of high dye stability as indicated in (Matcjeck German Patent 3,520,895). Therefore, it is not clear as to whether the polymer needs to remain in the coupler particle or just in the photographic layer to produce the observed dye stability.
In U.S. Pat. No. 4,490,461--Webb et al describes a process of dispersion preparation by homogenization of a solid solution of a photographic component and a polymer into aqueous gelatin solution by milling procedures. In the process of this invention, a photographic agent and a polymer is dissolved in a solvent. The solvent is then evaporated off to obtain a solid solution. The solid solution is then dispersed in aqueous gelatin by conventional milling procedures. In a specific embodiment this photographic compound is cross-linked to this polymer. This, in some cases is done by a cross-linking agent. The cross-linking may be done via a carboxyl group pendent on the polymer molecule. It is also known that conventional dispersion of photographic couplers can be prepared with some photographic advantages that contain both coupler solvent and a synthetic polyacrylamide polymer (U.S. Pat. No. 4,120,725--Nakazyo et al). In an alternate embodiment of this invention some water soluble acrylamide polymers can be added in aqueous phase along with gelatin for achieving added stability. Surfactant-like polymers containing--SO.sub.3 H groups in phenol formaldehyde resins (U.S. Pat. No. 4,198,478 and U.S. Pat. No. 4,569,905) and in acrylate polymers (U.S. Pat. No. 4,291,113) have been used to stabilize milled conventional dispersions.
Other solvent loading techniques like Chen's (U.S. Pat. No. 4,599,363) have been described in Tokitou et al (U.S. Pat. No. 4,358,533 and U.S. Pat. No. 4,368,258). U.S. Pat. No. 4,358,533 describes a process and composition where a photographic material is loaded into a polymer particle by using a large volume of water miscible solvent comprising a polymerized oligomeric material. In a special embodiment, the oligomeric material is polymerized in the presence of the photographic component to form a latex loaded composition. The process of latex loading in U.S. Pat. No. 4,368,258 is quite similar to U.S. Pat. No. 4,199,363--Chen et al. U.S. Pat. No. 2,852,386--Tong describes a very inefficient method of loading of couplers into latex dispersion by stirring the coupler for long periods of time with the latex and filtering off the excess coupler. This procedure led to less than 1 g of coupler per 20 g of the latex polymer in many cases. U.K. 1,456,278 describes loading of ultraviolet radiation absorbing compounds into polymer resin by the use of both permanent and auxiliary solvents in the presence of gelatin.
Chen's (U.S. Pat. No. 4,199,363) process where coupler solubilization and latex swelling are done by a water miscible solvent alone has several disadvantages. The impregnation of latex by the coupler is achieved in the case of Chen by evaporative removal of the solvent. As Chen's method is a solvent shift method, it requires a large amount of water miscible (auxiliary) solvent. By Chen's process, the amount of solvent needed is between 15 to 20 times the weight of the coupler to be imbibed. This is a major drawback of Chen's procedure. In Chen's process the maximum loading is 3 parts coupler to 1 part polymer, whereas higher loading would be desirable. Chen's method requires at least 2% by weight of the monomers to be of the type that form a water soluble polymer. A process that does not have any such requirement would be desirable.